Re: [PATCH v2] BaseTools/GenFw: Add X64 GOTPCREL Support to GenFw

[Andrew Fish <afish@apple.com>]

Zenith,

Stupid question, I'm not trying to derail the code review.....

In a static lib world like EFI, I can't figure out why we need support for GOT PC REL addressing modes? I say that as I would think they would be relocation entries in object files, and then the final link would happen and these relocation types would not be required in the final executable? Seems like the linker, at link time, would know the relative offset for all the static libraries that got linked together (edk2 build) and there is no need for a relocation. 

So I want to make sure this is not an issue caused by the wrong linker flags (emitting relocation for dynamic libs that is not supported in EF)? I'll freely admit there could be some circumstances I don't understand that require this support, but I just want to make sure we understand why we need to do it. Or maybe I'm missing something fundamental? 

Thanks,

Andrew Fish

> On Jun 7, 2018, at 5:49 AM, Zenith432 <zenith432@users.sourceforge.net> wrote:
> 
> Adds support for the following X64 ELF relocations to GenFw
>  R_X86_64_GOTPCREL
>  R_X86_64_GOTPCRELX
>  R_X86_64_REX_GOTPCRELX
> 
> CC: Shi Steven <steven.shi@intel.com>
> CC: Yonghong Zhu <yonghong.zhu@intel.com>
> CC: Liming Gao <liming.gao@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Zenith432 <zenith432@users.sourceforge.net>
> ---
> BaseTools/Source/C/GenFw/Elf64Convert.c | 166 +++++++++++++++++++++++-
> BaseTools/Source/C/GenFw/elf_common.h   |  23 ++++
> 2 files changed, 188 insertions(+), 1 deletion(-)
> 
> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c b/BaseTools/Source/C/GenFw/Elf64Convert.c
> index c39bdff0..1518c395 100644
> --- a/BaseTools/Source/C/GenFw/Elf64Convert.c
> +++ b/BaseTools/Source/C/GenFw/Elf64Convert.c
> @@ -94,6 +94,15 @@ STATIC Elf_Ehdr *mEhdr;
> STATIC Elf_Shdr *mShdrBase;
> STATIC Elf_Phdr *mPhdrBase;
> 
> +//
> +// GOT information
> +//
> +STATIC Elf_Shdr *mGOTShdr = NULL;
> +STATIC UINT32   mGOTShindex = 0;
> +STATIC UINT32   *mGOTCoffEntries = NULL;
> +STATIC UINT32   mGOTMaxCoffEntries = 0;
> +STATIC UINT32   mGOTNumCoffEntries = 0;
> +
> //
> // Coff information
> //
> @@ -322,6 +331,117 @@ GetSymName (
>   return StrtabContents + Sym->st_name;
> }
> 
> +//
> +// Find the ELF section hosting the GOT from an ELF Rva
> +//   of a single GOT entry.  Normally, GOT is placed in
> +//   ELF .text section, so assume once we find in which
> +//   section the GOT is, all GOT entries are there, and
> +//   just verify this.
> +//
> +STATIC
> +VOID
> +FindElfGOTSectionFromGOTEntryElfRva (
> +  Elf64_Addr GOTEntryElfRva
> +  )
> +{
> +  UINT32 i;
> +  if (mGOTShdr != NULL) {
> +    if (GOTEntryElfRva >= mGOTShdr->sh_addr &&
> +        GOTEntryElfRva <  mGOTShdr->sh_addr + mGOTShdr->sh_size)
> +      return;
> +    Error (NULL, 0, 3000, "Unsupported", "FindElfGOTSectionFromGOTEntryElfRva: GOT entries found in multiple sections.");
> +    exit(EXIT_FAILURE);
> +  }
> +  for (i = 0; i < mEhdr->e_shnum; i++) {
> +    Elf_Shdr *shdr = GetShdrByIndex(i);
> +    if (GOTEntryElfRva >= shdr->sh_addr &&
> +        GOTEntryElfRva <  shdr->sh_addr + shdr->sh_size) {
> +      mGOTShdr = shdr;
> +      mGOTShindex = i;
> +      return;
> +    }
> +  }
> +  Error (NULL, 0, 3000, "Invalid", "FindElfGOTSectionFromGOTEntryElfRva: ElfRva 0x%016LX for GOT entry not found in any section.", GOTEntryElfRva);
> +  exit(EXIT_FAILURE);
> +}
> +
> +//
> +// Stores locations of GOT entries in COFF image.
> +//   Returns TRUE if GOT entry is new.
> +//   Simple implementation as number of GOT
> +//   entries is expected to be low.
> +//
> +
> +STATIC
> +BOOLEAN
> +AccumulateCoffGOTEntries (
> +  UINT32 GOTCoffEntry
> +  )
> +{
> +  UINT32 i;
> +  if (mGOTCoffEntries != NULL) {
> +    for (i = 0; i < mGOTNumCoffEntries; i++)
> +      if (mGOTCoffEntries[i] == GOTCoffEntry)
> +        return FALSE;
> +  }
> +  if (mGOTCoffEntries == NULL) {
> +    mGOTCoffEntries = (UINT32*)malloc(5 * sizeof *mGOTCoffEntries);
> +    if (mGOTCoffEntries == NULL) {
> +      Error (NULL, 0, 4001, "Resource", "memory cannot be allocated!");
> +    }
> +    assert (mGOTCoffEntries != NULL);
> +    mGOTMaxCoffEntries = 5;
> +    mGOTNumCoffEntries = 0;
> +  } else if (mGOTNumCoffEntries == mGOTMaxCoffEntries) {
> +    mGOTCoffEntries = (UINT32*)realloc(mGOTCoffEntries, 2 * mGOTMaxCoffEntries * sizeof *mGOTCoffEntries);
> +    if (mGOTCoffEntries == NULL) {
> +      Error (NULL, 0, 4001, "Resource", "memory cannot be allocated!");
> +    }
> +    assert (mGOTCoffEntries != NULL);
> +    mGOTMaxCoffEntries += mGOTMaxCoffEntries;
> +  }
> +  mGOTCoffEntries[mGOTNumCoffEntries++] = GOTCoffEntry;
> +  return TRUE;
> +}
> +
> +STATIC
> +int
> +__comparator (
> +  const void* lhs,
> +  const void* rhs
> +  )
> +{
> +  if (*(const UINT32*)lhs < *(const UINT32*)rhs)
> +    return -1;
> +  return *(const UINT32*)lhs > *(const UINT32*)rhs;
> +}
> +
> +STATIC
> +VOID
> +EmitGOTRelocations (
> +  VOID
> +  )
> +{
> +  UINT32 i;
> +  if (mGOTCoffEntries == NULL)
> +    return;
> +  qsort(
> +    mGOTCoffEntries,
> +    mGOTNumCoffEntries,
> +    sizeof *mGOTCoffEntries,
> +    __comparator);
> +  for (i = 0; i < mGOTNumCoffEntries; i++) {
> +    VerboseMsg ("EFI_IMAGE_REL_BASED_DIR64 Offset: 0x%08X", mGOTCoffEntries[i]);
> +    CoffAddFixup(
> +      mGOTCoffEntries[i],
> +      EFI_IMAGE_REL_BASED_DIR64);
> +  }
> +  free(mGOTCoffEntries);
> +  mGOTCoffEntries = NULL;
> +  mGOTMaxCoffEntries = 0;
> +  mGOTNumCoffEntries = 0;
> +}
> +
> //
> // Elf functions interface implementation
> //
> @@ -698,7 +818,7 @@ WriteSections64 (
>     // section that applies to the entire binary, and which will have its section
>     // index set to #0 (which is a NULL section with the SHF_ALLOC bit cleared).
>     //
> -    // In the absence of GOT based relocations (which we currently don't support),
> +    // In the absence of GOT based relocations,
>     // this RELA section will contain redundant R_xxx_RELATIVE relocations, one
>     // for every R_xxx_xx64 relocation appearing in the per-section RELA sections.
>     // (i.e., .rela.text and .rela.data)
> @@ -780,6 +900,7 @@ WriteSections64 (
>         // Determine how to handle each relocation type based on the machine type.
>         //
>         if (mEhdr->e_machine == EM_X86_64) {
> +          Elf64_Addr GOTEntryRva;
>           switch (ELF_R_TYPE(Rel->r_info)) {
>           case R_X86_64_NONE:
>             break;
> @@ -834,6 +955,32 @@ WriteSections64 (
>               - (SecOffset - SecShdr->sh_addr));
>             VerboseMsg ("Relocation:  0x%08X", *(UINT32 *)Targ);
>             break;
> +          case R_X86_64_GOTPCREL:
> +          case R_X86_64_GOTPCRELX:
> +          case R_X86_64_REX_GOTPCRELX:
> +            VerboseMsg ("R_X86_64_GOTPCREL family");
> +            VerboseMsg ("Offset: 0x%08X, Addend: 0x%08X",
> +              (UINT32)(SecOffset + (Rel->r_offset - SecShdr->sh_addr)),
> +              *(UINT32 *)Targ);
> +            GOTEntryRva = Rel->r_offset - Rel->r_addend + *(INT32 *)Targ;
> +            FindElfGOTSectionFromGOTEntryElfRva(GOTEntryRva);
> +            *(UINT32 *)Targ = (UINT32) (*(UINT32 *)Targ
> +              + (mCoffSectionsOffset[mGOTShindex] - mGOTShdr->sh_addr)
> +              - (SecOffset - SecShdr->sh_addr));
> +            VerboseMsg ("Relocation:  0x%08X", *(UINT32 *)Targ);
> +            GOTEntryRva += (mCoffSectionsOffset[mGOTShindex] - mGOTShdr->sh_addr);  // ELF Rva -> COFF Rva
> +            if (AccumulateCoffGOTEntries((UINT32)GOTEntryRva)) {
> +              //
> +              // Relocate GOT entry if it's the first time we run into it
> +              //
> +              Targ = mCoffFile + GOTEntryRva;
> +              VerboseMsg ("Offset: 0x%08X, Addend: 0x%016LX",
> +                (UINT32)GOTEntryRva,
> +                *(UINT64 *)Targ);
> +              *(UINT64 *)Targ = *(UINT64 *)Targ - SymShdr->sh_addr + mCoffSectionsOffset[Sym->st_shndx];
> +              VerboseMsg ("Relocation:  0x%016LX", *(UINT64*)Targ);
> +            }
> +            break;
>           default:
>             Error (NULL, 0, 3000, "Invalid", "%s unsupported ELF EM_X86_64 relocation 0x%x.", mInImageName, (unsigned) ELF_R_TYPE(Rel->r_info));
>           }
> @@ -972,6 +1119,9 @@ WriteRelocations64 (
>             case R_X86_64_NONE:
>             case R_X86_64_PC32:
>             case R_X86_64_PLT32:
> +            case R_X86_64_GOTPCREL:
> +            case R_X86_64_GOTPCRELX:
> +            case R_X86_64_REX_GOTPCRELX:
>               break;
>             case R_X86_64_64:
>               VerboseMsg ("EFI_IMAGE_REL_BASED_DIR64 Offset: 0x%08X", 
> @@ -1040,10 +1190,24 @@ WriteRelocations64 (
>             Error (NULL, 0, 3000, "Not Supported", "This tool does not support relocations for ELF with e_machine %u (processor type).", (unsigned) mEhdr->e_machine);
>           }
>         }
> +        if (mEhdr->e_machine == EM_X86_64 && RelShdr->sh_info == mGOTShindex) {
> +          //
> +          // Tack relocations for GOT entries after other relocations for
> +          //   the section the GOT is in, as it's usually found at the end
> +          //   of the section.
> +          //
> +          EmitGOTRelocations();
> +        }
>       }
>     }
>   }
> 
> +  if (mEhdr->e_machine == EM_X86_64) {
> +    //
> +    // Just in case GOT is in a section with no other relocations
> +    //
> +    EmitGOTRelocations();
> +  }
>   //
>   // Pad by adding empty entries.
>   //
> diff --git a/BaseTools/Source/C/GenFw/elf_common.h b/BaseTools/Source/C/GenFw/elf_common.h
> index 766d0e42..50b4e1f2 100644
> --- a/BaseTools/Source/C/GenFw/elf_common.h
> +++ b/BaseTools/Source/C/GenFw/elf_common.h
> @@ -544,6 +544,12 @@ typedef struct {
> #define	R_386_TLS_DTPMOD32	35	/* GOT entry containing TLS index */
> #define	R_386_TLS_DTPOFF32	36	/* GOT entry containing TLS offset */
> #define	R_386_TLS_TPOFF32	37	/* GOT entry of -ve static TLS offset */
> +#define	R_386_SIZE32		38	/* 32-bit symbol size */
> +#define	R_386_TLS_GOTDESC	39	/* GOT offset for TLS descriptor. */
> +#define	R_386_TLS_DESC_CALL	40	/* Marker of call through TLS descriptor for relaxation. */
> +#define	R_386_TLS_DESC		41	/* TLS descriptor containing pointer to code and to argument, returning the TLS offset for the symbol. */
> +#define	R_386_IRELATIVE		42	/* Adjust indirectly by program base */
> +#define	R_386_GOT32X		43	/* Load from 32 bit GOT entry, relaxable. */
> 
> /* Null relocation */
> #define	R_AARCH64_NONE				256	/* No relocation */
> @@ -1052,6 +1058,23 @@ typedef struct {
> #define	R_X86_64_DTPOFF32	21	/* Offset in TLS block */
> #define	R_X86_64_GOTTPOFF	22	/* PC relative offset to IE GOT entry */
> #define	R_X86_64_TPOFF32	23	/* Offset in static TLS block */
> +#define	R_X86_64_PC64		24	/* PC relative 64 bit */
> +#define	R_X86_64_GOTOFF64	25	/* 64 bit offset to GOT */
> +#define	R_X86_64_GOTPC32	26	/* 32 bit signed pc relative offset to GOT */
> +#define	R_X86_64_GOT64		27	/* 64-bit GOT entry offset */
> +#define	R_X86_64_GOTPCREL64	28	/* 64-bit PC relative offset to GOT entry */
> +#define	R_X86_64_GOTPC64	29	/* 64-bit PC relative offset to GOT */
> +#define	R_X86_64_GOTPLT64	30	/* like GOT64, says PLT entry needed */
> +#define	R_X86_64_PLTOFF64	31	/* 64-bit GOT relative offset to PLT entry */
> +#define	R_X86_64_SIZE32		32	/* Size of symbol plus 32-bit addend */
> +#define	R_X86_64_SIZE64		33	/* Size of symbol plus 64-bit addend */
> +#define	R_X86_64_GOTPC32_TLSDESC	34	/* GOT offset for TLS descriptor. */
> +#define	R_X86_64_TLSDESC_CALL	35	/* Marker for call through TLS descriptor. */
> +#define	R_X86_64_TLSDESC	36	/* TLS descriptor. */
> +#define	R_X86_64_IRELATIVE	37	/* Adjust indirectly by program base */
> +#define	R_X86_64_RELATIVE64	38	/* 64-bit adjust by program base */
> +#define	R_X86_64_GOTPCRELX	41	/* Load from 32 bit signed pc relative offset to GOT entry without REX prefix, relaxable. */
> +#define	R_X86_64_REX_GOTPCRELX	42	/* Load from 32 bit signed pc relative offset to GOT entry with REX prefix, relaxable. */
> 
> 
> #endif /* !_SYS_ELF_COMMON_H_ */
> -- 
> 2.17.1
> 
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